(1) Field of the Invention
The present invention relates to an optical subscriber line terminal unit and a state transition control method, and more particularly, to an optical subscriber line terminal unit connected to an optical subscriber system for performing optical burst transmission and to a state transition control method for controlling the operation of state transition at the time of startup.
(2) Description of the Related Art
In recent years, communication services are exponentially diversifying and demand for video-on-demand, CATV, high-speed computer communication, etc. is expanding. To provide such bulk communication services at low price, it is essential to use an optical communication system in which a subscriber communication network is constructed using optical devices.
As such optical communication system, PDS (Passive Double Star) has been proposed in which one optical fiber is shared by a plurality of users. Also, PON (Passive Optical Network) system using the PDS technique is attracting attention especially in Europe, and FTTH (Fiber To The Home) system in which an optical fiber is laid up to each home is under development for realization.
To realize such FTTH system, ATM-PON using ATM (Asynchronous Transfer Mode) to reliably provide communication band and quality in response to real-time requests for communications of voice, moving images, etc. is being constructed under the leadership of FSAN (Full Service Access Networks: an international association of communication industries established to promote optical communication business).
FIG. 8 shows the configuration of an ATM-PON system. ONUs (Optical Network Units: optical subscriber line terminal units) 101a to 101n for performing optical burst transmission are arranged at respective subscribers"" homes 100a to 100n, and an OLT (Optical Line Terminal: optical subscriber line terminal unit) 201 is arranged in an office 200.
Telephones, CATVs, etc. are connected to the ONUs 101a to 101n, and an exchange (ATM exchange, ISDN exchange, etc.) 202 is connected to the OLT 201. Also, the ONUs 101a to 101n and the OLT 201 are connected to a star coupler 300.
Down data (down cells) directed from the office 200 to the subscribers"" homes 100a to 100n is transmitted through a single optical fiber, then the star coupler 300, and a plurality of optical fibers diverging from the star coupler like branches. Up data (up cells) directed from the subscribers"" homes 100a to 100n to the office 200 is transmitted through the respective branch optical fibers, the star coupler 300, and the concentrated single optical fiber.
Thus, the ATM-PON system is an optical branch type access network using ATM wherein an office and a plurality of subscribers are connected in the ratio 1:n by the star coupler 300.
According to the ITU-T recommendation G. 983, on the other hand, state o1 through state o10 are determined as the operation state of ONU, and it is recommended in particular that after the power is turned on, ONU should be in the o1 or o9 state.
The o1 state is an initial state of ONU after the power is turned on. The o9 state is an emergency stop state in which ONU is disconnected from the network and is incapable of communication.
If, for example, a malicious user enters the network using an unregistered ONU, the OLT requests the ONU by its serial number to transmit a password (each ONU is assigned a serial number and a password), and if it is found that there is a discrepancy between the serial number and the password (the OLT is informed in advance of the serial numbers of ONUs and their corresponding passwords), the OLT sends a message to bring the ONU to the o9 state. On receiving the message, the ONU shifts to the o9 state and is brought to an emergency stop.
Once the ONU is brought to the o9 state, it is unable to escape from the o9 state (an o9 state flag is set in the firmware inside the ONU) even if it is switched on again after the power is cut off, unless the ONU is supplied with an o9 state cancellation message from the OLT, thereby making it impossible for a malicious user to use again the ONU.
If the user is thereafter identified as an authorized user by a system administrator and the ONU is registered, the OLT sends an o9 state cancellation message. On receiving the message, the ONU shifts from the o9 state to the o1 state and performs a normal startup operation.
In the ITU-T recommendation G. 983, however, there is no provision as to the sequence process for determining whether the ONU should shift to the o1 state or the o9 state as the operation state after the power is turned on.
Accordingly, when an ONU which has been brought to the o9 state is switched on again after the power is cut off, the ONU may possibly perform an unnatural operation such that it once shifts to the o1 state from a clear state at the start of power supply, and then to the o9 state, thus giving rise to a problem that the startup operation is unstable.
Also, if, during normal operation of the ONU, the o9 state flag is erroneously set for some firmware-related reason, the ONU fails to perform normal operation of state transition, resulting in erroneous operation.
One object of the present invention is to provide an optical subscriber line terminal unit which is stabilized in operation.
To achieve the above object, there is provided an optical subscriber line terminal unit connected to an optical subscriber system for performing optical burst transmission. The optical subscriber line terminal unit comprises state information storing means for storing state information about the unit, startup preparatory state shifting means for causing a shift to a startup preparatory state at startup, flag setting means for setting an emergency stop state flag during a period of the startup preparatory state if it is judged based on the state information that a state before the startup is an emergency stop state, and state transition control means for causing a shift from the startup preparatory state to the emergency stop state if the emergency stop state flag is set, and causing a shift from the startup preparatory state to an initial state if the emergency stop state flag is not set.
Another object of the present invention is to provide a state transition control method for stabilizing operation.
To achieve the object, there is provided a state transition control method for controlling operation of state transition at startup. The state transition control method comprises storing state information about a terminal unit in a nonvolatile memory, causing a shift to a startup preparatory state at startup, setting an emergency stop state flag during a period of the startup preparatory state if it is judged based on the state information that a state before the startup is an emergency stop state, causing a shift from the startup preparatory state to the emergency stop state if the emergency stop state flag is set, and causing a shift from the startup preparatory state to an initial state if the emergency stop state flag is not set.